I. Field of the Invention
This invention relates to a process which utilizes supported, non (metal-promoted) carbon-containing molybdenum sulfide catalysts for the selective removal of nitrogen in the processing of lube oil base stocks and distillates.
II. Background and Prior Art
Lube oils and specialty oils are made by blending together lube base stocks and additives. Lube base stocks are obtained by distillation from a crude hydrocarbon admixture, the lube distillate meeting the required viscosity and boiling range specifications. Chemical additives are then used to enhance certain properties of the lube base stocks or to add characteristics not present in the lube base stocks.
These oils differ widely in quality, and are selected to serve specific uses, dependent on the type of service to which the oil is to be subjected. Quality differences are achieved in large part by a selection of crude oil source, type of crude oil, and the viscosity of the lube distillate fraction. Motor and aviation oils, e.g., must show the least possible change in viscosity with temperature. They must provide equally good lubrication at cold start-up and at high operating temperatures. Industrial oils must be stable over long periods, though change in viscosity with temperature is less critical than with motor and aviation oils. Specialty oils, on the other hand, require properties other than, or in addition to, those of providing good lubrication. Thus, e.g., transformer oils, medicinal white oils and hydraulic fluids differ widely in their required properties.
Lube oil distillates, obtained from fractions taken from a vacuum pipestill, are constituted principally of paraffins, isoparaffins, naphthenes and aromatics. The predominant molecular structures in these distillates are naphthene ring compounds, especially naphthene-aromatic compounds containing from one to about six rings to which paraffin chains are attached. Lube oil distillates also contain sulfur, and nitrogen as impurities. Normal paraffins are generally removed from the oils during processing. Oxygen can lead to the production of oxidation products under the conditions at which internal combustion engines are operated, these products often being acidic in nature and highly corrosive. Oxidation products also form polymeric materials, and act as sludge binders which hold together the water of combustion, lead salts, carbon and road dust which may impair ring performance, and plug oil lines and filters.
Sulfur can also lead to the formation of oxidation products. Much of the sulfur contained in the lube oil distillate, however, is contained inside the ring structures of compounds which impart desirable lubricity and stability characteristics to the lube oil, and accordingly removal of this type of sulfur from the lube oil is undesirable. Chemical additions, for this reason, are often made to impart to the lube oil resistance to oxidation and thermal degradation of the lubricants. These additives, in fact, often include sulfur compounds such as the sulfurized terpenes, phosphosulfurized materials such as the phosphosulfurized terpenes and the zinc dialkyldithiophosphonates. Nitrogen, on the other hand, can be detrimental in lube oils. It is therefore desirable to remove as much of the nitrogen as possible from a lube oil distillate before it is blended to form a lube base stock.
Hydrocarbon oils are generally hydrotreated, this removing sulfur and nitrogen from the oil. The removal of nitrogen is desirable and, as suggested, the removal of a significant amount of sulfur is undesirable and even detrimental in the manufacture of lube base stocks. Conventional hydrotreating catalysts offer poor flexibility with regard to nitrogen/sulfur selectivity, and the vast preponderance of these catalysts are very highly selective for the removal of sulfur, more so generally than for the removal of nitrogen. Most thus remove both sulfur and nitrogen, generally more sulfur than nitrogen. Only a modest increase is possible in nitrogen selectivity vis-a-vis sulfur selectivity by using, e.g., conventional nickel-molybdenum-on-alumina catalysts as opposed to conventional cobalt-molybdenum-on-alumina catalysts, or by operating at abnormally elevated pressures, but the range of selectivity control is quite limited.
Supported carbon-containing molybdenum sulfide and tungsten sulfide catalysts such as disclosed in application Ser. Nos. 400,004 and 400,005, by Robert L. Seiver and Russell R. Chianelli, both filed July 20, 1982, have been found useful as hydrotreating and methanation catalysts. These catalysts are prepared from precursors formed by supporting on a porous, refractory inorganic oxide carrier, a complex salt characterized by the formula B.sub.x [MO.sub.y S.sub.4-y ] where B is an organo or hydrocarbyl substituted diammonium ion, an organo or hydrocarbyl substituted ammonium ion or quaternary ammonium ion, or an ionic form of a cyclic amine containing one or more basic N atoms; x is 1 where B is an organo or hydrocarbyl substituted diammonium ion, or 2 where B is an organo or hydrocarbyl substituted ammonium or quaternary ammonium ion or an ionic form of a cyclic amine containing one or more basic N atoms; M is molybdenum or tungsten; and y is 0, or a fraction or whole number ranging up to 3. The catalysts are formed by heat decomposing the salt of said catalyst precursor composite (1) in the presence of hydrogen, hydrocarbon and sulfur, or (2) in the presence of hydrogen and hydrogen sulfide. The precise nature, and composition of the catalyst species that is formed on the support as a reaction product of the decomposition reaction is not known, but it is believed that a catalyst species having the general formula MS.sub.2-z C.sub.z ', wherein M is molybdenum or tungsten, and z and z' are the same or different and range from about 0.01 to about 0.5. The surface composition, or composition deposited on the surface of the support, is believed to correspond generally with the unsupported catalyst species defined in application Ser. Nos. 399,999 and 399,991, each jointly filed by Theresa R. Pecoraro and Russell R. Chianelli, and Russell R. Chianelli and Theresa R. Pecoraro, respectively, also on July 20, 1982. As a class these catalysts are active for the hydrodenitrogenation (HDN) and the hydrodesulfurization (HDS) of hydrocarbon feedstocks which contain relatively high concentrations of sulfur, or nitrogen, or both. The catalysts hydrodenitrogenate hydrocarbon feeds, and have offered a considerably wider range of HDN/HDS selectivity than conventional prior art catalysts.